The overall vaccination rate in the United States is still high, fortunately, despite this worrisome trend. For example, according to the Centers for Disease Control and Prevention, more than 90 percent of 19- to 35-month-old American children are adequately vaccinated against measles and chickenpox. Why, then, do we continue to see outbreaks of diseases preventable in the United States?

One reason is the epidemiological phenomenon of clustering of susceptible individuals — which happens when a group of unvaccinated individuals in a specific area grows large enough to render protection from overall high immunization rates less effective.

The ongoing chickenpox outbreak at a Waldorf school in Asheville, N.C., is the most recent example of this problem. The outbreak is the largest chickenpox epidemic in North Carolina in over two decades. Waldorf schools emphasize “independent and inclusive” education as well as academic rigor, and alternative educational institutions such as Waldorf have much higher rates of vaccine refusal than public schools and other private schools. In a study of California schools, we found that Waldorf schools had vaccine refusal rates that were 19 times as high as those at public schools.

Alternative schools, specifically Waldorf schools, have been associated with outbreaks outside the United States as well. Many recent outbreaks of measles and other preventable diseases in the United States have started among geographic clusters of vaccine refusers. Outbreaks in clusters of vaccine refusers mean that public health authorities and doctors need to focus on these areas of vulnerability, not just on increasing overall vaccination rates.

Vaccines protect communities not only through direct protection of vaccinated individuals if they encounter viruses or bacteria, but also by reducing the likelihood of encountering these infections. Because vaccinated individuals have a substantially lower risk of catching and, therefore, passing on the infection, there are simply fewer chances for anyone to get it. When enough people are vaccinated, the risk of outbreaks goes down; when the vaccination rate gets high enough, the outbreak risk is virtually eliminated. This phenomenon is known as herd immunity (or community immunity), and the immunization rate at which the risk of outbreaks is eliminated is called the herd immunity threshold. That threshold depends on the infectivity of a virus or bacterium and, therefore, varies by disease.

Herd immunity is important for several reasons. First, not everyone can be vaccinated: Children with certain medical conditions, such as some types of immunodeficiencies, and those on cancer chemotherapy cannot receive most vaccines, for instance. These vulnerable individuals, particularly children, need herd immunity to protect them from diseases that vaccines can prevent. And because few vaccines are 100 percent effective, some vaccinated children remain individually unprotected and rely on herd immunity, as well. Those with chronic medical conditions such as asthma or diabetes are particularly vulnerable to severe outcomes associated with vaccine-preventable diseases. For instance, people with diabetes are six times as likely to be hospitalized for an influenza-related reason as those without diabetes and are three times as likely to die because of influenza infection. Similarly, individuals with asthma have a higher risk of whooping cough.

Clusters of vaccine refusal breach the herd immunity threshold in specific schools or neighborhoods — which means outbreaks can occur even if overall vaccination rates are high.

Equally important, in an analysis of U.S. measles outbreaks since 2000, we found that vaccine refusers are disproportionately represented in early stages of outbreaks. People who deliberately go unvaccinated can provide the critical mass of susceptible individuals that can help start outbreaks that vaccination would otherwise have prevented.

While we do not completely understand why vaccine refusal clusters develop, the evidence points to aggregation based on shared values, demographics and socioeconomic status. School districts vary in how they implement immunization mandates as well, as our research has found.

One thing is clear: These clusters occur in both left- and right-leaning communities. For example, Vashon Island, Wash., home to many affluent “ex-hippies,” has had notoriously high vaccine-refusal rates. On the other hand, some of the highest rates of vaccine refusal in Washington state are in rural Ferry County, which voted for Donald Trump by a margin of approximately 30 percentage points.

Most public health and policy interventions do not explicitly focus on addressing clustering of vaccine refusers. But some potential approaches can target this problem. For example, state health departments could ensure that all schools appropriately implement the school immunization requirements. Community education and communication efforts could target communities with high rates of vaccine refusal. In all interventions to increase vaccine acceptance, health-care providers should be front and center, as research shows that they are the most trusted source of vaccine information, even among those who refuse vaccines. Evidence-based state-level interventions can reduce local clustering as well. For example, Washington state recently enacted a law that mandates health-care-provider counseling as a condition for exemption from school-level immunization requirements. This law not only resulted in a reduction of more than 40 percent in state-level vaccine exemption rates, but there was substantial reduction in clustering of vaccine refusal. At the individual level, parents should demand that states and schools share the rates of vaccine refusal in their schools and communities publicly.

Clustering of vaccine refusers increases the likelihood of outbreaks beyond what would be expected from overall high vaccination rates. As the vaccine-refusal rate inches up, public health authorities, physicians and parents need to pay attention to clustering as well as overall rate of vaccine refusal. Otherwise, we are likely to continue to see more and larger outbreaks.